Cathode with improved work function and method for making the same

ABSTRACT

A cathode with an improved work function, for use in a lithographic system, such as the SCALPEL™ system, which includes a buffer between a substrate and an emissive layer, where the buffer alters, randomizes, miniaturizes, and/or isolates the grain structure at a surface of the substrate to reduce the grain size, randomize crystal orientation and reduce the rate of crystal growth. The buffer layer may be a solid solution or a multiphase alloy. A method of making the cathode by depositing a buffer between a surface of the substrate and an emissive layer, where the deposited buffer alters, randomizes, miniaturizes, and/or isolates the grain structure at a surface of the substrate to reduce the grain size, randomize crystal orientation and reduce the rate of crystal growth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cathode for use in electron beamprojection lithography and a method for making the cathode, and moreparticularly, to a cathode with an improved work function and a methodfor making the improved work function cathode.

2. Description of the Related Art

Projection electron beam lithography, such as Scattering AngularLimitation Projection Electron Beam Lithograph (SCALPEL™), utilizeselectron beam radiation projected onto a patterned mask to transfer animage of that pattern into a layer of energy sensitive material formedon a substrate. That image is developed and used in subsequentprocessing to form devices such as integrated circuits.

The SCALPEL™ mask has a membrane of a low atomic number material onwhich is formed a layer of high atomic number material. The layer ofhigh atomic number material has a pattern delineated therein. Both thelow atomic number membrane material and the high atomic number patternedlayer of material are transparent to the electrons projected thereon(i.e., electrons with an energy of about 100 keV). However, the lowatomic number membrane materials scatters the electrons weakly and atsmall angles. The high atomic number patterned layer of materialscatters the electrons strongly and at large angles. Thus, the electronstransmitted through the high atomic number patterned material have alarger scattering angle than the electrons transmitted through themembrane. This difference in scattering angle provides a contrastbetween the electrons transmitted through the membrane alone and theelectrons transmitted through the layer of patterned material formed onthe membrane.

This contrast is exploited to transfer an image of the pattern from themask and into a layer of energy sensitive material by using a back focalplane filter in the projection optics between the mask and the layer ofenergy sensitive material. The back focal pane filter has an aperturetherein. The weakly scattered electrons are transmitted through theaperture while the strongly scattered electrons are blocked by the backfocal plane filter. Thus, the image of the pattern defined in the weaklyscattered electrons is transmitted through the aperture and into thelayer of energy sensitive material.

FIG. 1 is a schematic diagram illustrating the concept of a conventionalSCALPEL™ system. A beam B of electrons 10 is directed towards ascattering mask 9 including a thin membrane 11 having a thicknessbetween about 1,000 Å and about 20,000 Å (0.1 μm and about 2 μm thick.)The membrane 11 is composed of a material which is virtually transparentto the electron beam B composed of electrons 10. That is to say thatelectrons 10 in beam B pass through membrane 11 freely in the absence ofany other object providing an obstruction to the path of electrons 10 inthe beam B as they pass from the source of the beam through the membrane11.

Formed on the side of the membrane 11 facing the beam 10, is a patternof high density scattering elements 12 to provide a contrast mechanismthat enables reproduction of the mask pattern at the target surface. Thescattering elements 12 are patterned in the composite shape which is tobe exposed upon a work piece 17 (usually a silicon wafer) which iscoated with E-beam sensitive resist, which as shown in FIG. 1 has beenprocessed into pattern elements 18. The electrons 10 from the E-beam Bwhich pass through the mask 9 are shown by beams 14 which pass throughelectromagnetic lens 15 which focuses the beams 14 through an aperture16′ into an otherwise opaque back focal plane filter 16. The aperture16′ permits only electrons scattered at small angles to pass through tothe work piece 17.

A conventional SCALPEL™ exposure tool is illustrated in FIG. 2. Theexposure tool 20 includes a source 22 (usually an electron gun), a maskstage 24, imaging optics 26, and a wafer stage 28. The mask stage 24 andthe wafer stage 28 are mounted to the top and bottom of a block ofaluminum, referred to as the metrology plate 30. The metrology plate 30,which is on the order of 3000 lbs., serves as a thermal and mechanicalstabilizer for the entire exposure tool 20.

FIG. 3 illustrates a prior art source 22 in more detail. The source 22includes a cathode 42, an anode 43, a grid electrode 44, focusing plates45, and a filament 46. Each of the cathode 42, anode 43, grid electrode44, and focusing plates 45 exhibit substantial circular and radialsymmetry about an imaginary line of focus 50. In the prior art systemsin U.S. Pat. No. 5,426,686, the cathode 42 is made of gallium arsenide(GaAs), bialkali cathode materials, cesium antimonide (Cs₃Sb), or a purematerial having a low work function, such as tantalum (Ta), samarium(Sm), or nickel (Ni). In other prior art systems disclosed in U.S. Pat.No. 5,426,686, the material of photocathode 42 is made of a metal addedto a semiconductor material by mixing or by depositing throughsputtering or annealing. The metal is preferably tantalum (Ta), copper(Cu), silver (Ag), aluminum (Al), or gold (Au), or oxides or halides ofthese metals. One such example of a prior art photocathode isconstructed from tantalum (Ta) annealed on the surface of nickel (Ni).

Most e-beam lithography systems (direct e-beam writing machines, etc.)require essentially point electron sources with high current densities.Conventional thermoionic cathodes, such as pure metal (tungsten ortantalum), lanthanum hexaboride (LaB₆), etc. cathodes are sufficient forthese applications.

In contrast, SCALPEL™ systems require a 1 mm² approximately parallelelectron beam with a cross-sectional current density variation of within2%. Conventional thermoionic cathodes have work function variationsacross the emitting surface substantially greater than 2%, for example5-10%. However, as noted on page 3769 of “High emittance electron gunfor projection lithography,” W. Devore et al., 1998 American VacuumSociety, J. Vac. Sci. Technol. B 14(6), November/December 1996, pp.3764-3769, the SCALPEL™ process requires a thermoionic cathode with awork function variation less than 2%.

The conventional cathode which meets the SCALPEL™ requirements for otherparameters, such as emission uniformity, low work function, lowevaporation rate, high voltage operating environment, and vacuumcontamination resilience is a tantalum (Ta) cathode having a disk shape.The disk-shaped tantalum (Ta) cathode is made from cold or hot rolledtantalum (Ta) foils which are hot pressed into a micro-polycrystallinematerial. Because of its polycrystalline nature, the grains aresubstantially misoriented with each other (on the order of 5-20°). Theconventional Ta cathode also has an uncontrolled grain size distributionbetween 5-400 μm. Due to the sensitivity of tantalum's work function tothe crystallographic orientation, the conventional polycrystalline Tacathode work function distribution is “patchy” (also on the order of5-400 μm), varying from grain-to-grain (because of differingorientations) and resulting in an unacceptably patchy or non-uniformemission pattern. Growth of the misoriented and differing sized grainsat a high operating temperature further aggravates the patchinessproblem. The non-uniformities caused by grain misorientation,uncontrolled large grain sizes, and grain growth on the cathode surfaceat the high operating temperature are transferred by the SCALPEL™electron optics down to the shaping aperture (the object plane) andeventually to the wafer surface (the imaging plane).

When used as a SCALPEL™ cathode, the conventional polycrystallinecathode materials experience grain growth and rough texture development(together termed “recrystallization”) at the SCALPEL™ high operatingtemperatures (1200-2000° C.) and extended time period (greater than 1000hours). Although the total emission current is satisfactory, structuraldevelopments at the cathode surface causes dark patches to appear on thecathode surface and make the cathode unacceptable for SCALPEL™. Inaddition, conventional cathode materials, such as LaB₆, are easilycontaminated by the SCALPEL™ operating environment, as described in“Design of a low-brightness, highly uniform source for projectionelectron-beam lithography (SCALPEL™)”, W. K. Waskiewicz et al., Proc.SPIE, 3155 (1997).

SUMMARY OF THE INVENTION

The present invention solves these problems with conventional cathodesused in SCALPEL™ and similar systems by providing a cathode that has abuffer between a polycrystalline substrate and an emissive layer.

The work function of the conventional polycrystalline substrate surfaceis non-uniform due to the non-uniform grain crystallography of thesubstrate material at the surface. These non-uniformities include grainmisorientations on the order of 5-20° and grain size variations from5-400 μm. Recrystallization over time also results in an increase ingrain size and misorientation. The buffer alters, randomizes,miniaturizes (preferably to grain sizes less than 4 μm), and/or isolatesthe emissive layer, in a crystallographic sense, from the underlyingsubstrate. The buffer also reduces the rate at which the substrate andemissive layer recrystallize over time.

In an example where the cathode is a refractory metal or carbon cathode(e.g., a tantalum cathode) the buffer also includes a refractory metalor carbon.

In one example, the substrate is tantalum, the buffer is a dual layer ofmolybdenum and tungsten, and the emissive layer is tantalum. Themolybdenum modifies the tantalum structure (lattice constant andorientation) in the underlying substrate by dissolving into thesubstrate, which reduces grain misorientation. The tungsten alsodissolves into the substrate and, as a result of its high meltingtemperature, reduces the rate of recrystallization of both theunderlying tantalum substrate and the overlying tantalum emissive layer.In particular, the buffer reduces the rate at which the substrate andemissive layer recrystallize over time by at least 30% and preferably by50%.

In another example, rhenium and tantalum are codeposited to form thebuffer. The codeposit forms fine-grained (less than 4 μm) intermetallicphases and reduces subsequent recrystallization of the substrate andemissive layer. The codeposit does not adversely affect the electrontransport from the substrate to the emissive layer, and has acoefficient of thermal expansion that approximates (within 35%, morepreferably within 25%) that of the substrate and the emissive layer.

However, the codeposit interacts with the substrate in a way that isdifferent from the interaction between the substrate and the molybdenum.The codeposit does not dissolve and does not alter the originalstructure of the substrate but rather blocks and dominates the originalsubstrate surface. In effect, the structure of the codeposit dominatesthe substrate, composed of randomly oriented fine grains of Re—Taintermetallic phases (less than 4 μm) resulting in a uniformlydistributed, averaged work function. The end result however, is thesame; the emissive layer is effectively isolated from the substrate andnot affected by the crystal structure of the substrate surface.

In another example, the substrate is tantalum, the buffer is rhenium,and the emissive layer is tantalum. The rhenium modifies the tantalumstructure in the underlying substrate by reacting with the tantalum toform Re—Ta intermetallic phases, similar to those obtained with theRe—Ta codeposit and that minimizes the misorientation of the grains.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the concept of a SCALPEL™(Scattering Angle Limited Projection E-bean Lithography) system.

FIG. 2 illustrates a conventional SCALPEL™ exposure tool.

FIG. 3 illustrates a conventional source for the SCALPEL™ system of FIG.1.

FIG. 4 illustrates a general method of making the cathode of the presentinvention.

FIG. 5 illustrates the general cathode of the present invention.

FIGS. 6A-6C illustrate a cathode with a solid solution buffer region.

FIG. 7 illustrates a cathode with an alloy buffer film.

FIGS. 8A-8B illustrate another cathode with a solid solution bufferregion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 4 illustrates a general embodiment of the method of making thecathode of the present invention. As illustrated in FIG. 4, in step 100,a surface of a substrate is prepared for deposition. In step 200, abuffer is deposited on the substrate. In step 300, an emissive layer isdeposited on the buffer, in order to produce the cathode.

In more particular embodiments, the surface of the substrate is preparedby ion etching and vacuum annealing. Further, the buffer and/or theemissive layer are deposited by magnetron sputter deposition. Further,in preferred embodiments, vacuum annealing is performed after depositionof both the buffer and the emissive layer.

In other preferred embodiments, the buffer and layers may be depositedby a number of different methods, such as sputtering, evaporation,chemical vapor deposition (CVD), etc. The desired thickness of thebuffer is typically in the range of 0.1 μm to 100 μm, or preferably inthe range of 0.5-10 μm.

FIG. 5 illustrates a cathode of the present invention in one embodiment.In particular, FIG. 5 illustrates that the cathode 110 includes asubstrate 112, a buffer 114, an emissive layer 116, arranged asillustrated in FIG. 4.

In one embodiment, the buffer 114 is an altered grain structure at asurface of the substrate 112 to make the work function distribution moreuniform. In another embodiment, the buffer 114 is itself a fine-grained,randomized structure that blocks the substrate 112, thereby improvingthe uniformity of the work function distribution of the resultingcathode 110.

In more particular embodiments of the present invention, the buffer 114alters the grain structure at the surface of the substrate by diffusion,alloying, precipitation, or new phase formation. Still further, thebuffer 114 includes atoms from a chemical class similar to the chemicalclass of the substrate 112. For example, if the substrate 112 is arefractory metal or carbon, then the buffer 114 would also includerefractory metal or carbon atoms. In another preferred embodiment, thecathode is part of a projection electron lithography system, such as theSCALPEL™ system.

The buffer 114 also provides thermal and electrical conductivities andgood adhesion to withstand operating temperatures up to 2100° K. Oneadditional advantage of the cathode 110 illustrated in FIG. 5, is thatsuch a layered cathode can be made in a curved shape (concave orconvex), which is useful for electron beam focusing.

EXAMPLE 1

Example 1 describes a first material combination that is effective increating the desired structural uniformity and work function uniformityon the polycrystalline Ta cathode surface. Example 1 is a Ta/Mo/W/Taarrangement, where Ta is the polycrystalline substrate surface, Mo/W aretwo sequentially applied buffer layers, and Ta is the emissive layer.

In the Ta/Mo/W/Ta system, both Mo and W have the same body centeredcubic (bcc) structure as Ta, and form solid solutions. Because of therelatively small size of Mo atoms, the first buffer of Mo atoms diffuseinto the Ta substrate upon annealing at 1600° C. This Mo diffusionalters the crystalline structure of the existing Ta grains. Thesubsequent W layer further alters the grain structure on the surface bydiffusion to form a solid solution of Ta-Mo-W. The final Ta layer servesas the emissive layer because it has the lowest work function of thethree.

In FIG. 6A, an Mo layer 1142 is added to the Ta substrate 112. Becauseof the relatively small size of the Mo atoms, the Mo atoms diffuse intothe Ta substrate 112 upon annealing at 1600° C. This Mo diffusion altersor distorts the crystalline structure of the existing Ta grains to formTa/Mo region 1144, thereby randomizing the orientation of the Tasubstrate. Then, a W layer 1148 is added, as illustrated in FIG. 6B. TheW layer 1148 further randomizes the grain structure on the surface bydiffusion to form a solid solution of Ta/Mo/W at 1150. Finally, the Taemissive layer 116 is added, as illustrated in FIG. 6C. The final Talayer serves as the emissive layer, because it has the lowest workfunction of the three. The grain structure of the Ta layer 116 is alsorandomized because of the underlying Ta/Mo/W region 150.

The Mo layer 1142 and W layer 1148 are typically added at a thickness of0.5-10 μm. In other preferred examples, both the Mo layer 1142 and the Wlayer 1148 may be selected from the group including Mo, W, Nb, V, Ir,Rh, or any combination thereof.

EXAMPLE 2

Example 2 describes a second material combination that is effective increating the desired structural uniformity and work function uniformityon the polycrystalline Ta cathode surface. Example 2 is a Ta/Re—Ta/Taarrangement, where Ta is the polycrystalline substrate 112, Re—Ta is analloyed buffer film 113, and Ta is the emissive layer 116, asillustrated in FIG. 7.

Because Re has a hexagonal close-packed (hcp) structure, it formsvarious intermetallic compounds with Ta. The co-sputtered Re—Ta alloyfilm 113 thus consists of fine-grained (<4 μm), randomly orientedsurface structure, which blocks the original polycrystalline grainstructure of the Ta substrate 112. The subsequent emissive Ta layer 116also has the same fine grain structure because of the Re—Ta alloy film113. Because of the formation of intermetallic compounds and theresultant multiphase structure, the grains in the substrate 112 arelargely pinned by the new phases of the buffer film 113 at theboundaries and their growth is inhibited at the operating temperature(2100° K). Also Re has a close match with Ta in its thermal expansioncoefficient, and as a result, the buildup of thermal stress which causefilm delamination and cracking, is reduced.

Although the intermetallic compound of Example 2 is a Re—Ta alloy, othercombinations are also effective including C—Ta, Hf—Ta, Os—Ta, and Ru—Ta.

EXAMPLE 3

Example 3 describes a third material combination that is effective increating the desired structural uniformity and work function uniformityon the polycrystalline Ta cathode surface. Example 3 is a Ta/Re/Taarrangement, where Ta is the polycrystalline substrate 112, Re is abuffer layer 1160, and Ta is the emissive layer 116, as illustrated inFIG. 8.

In FIG. 8A, a Re layer 1160 is added to the Ta substrate 112. The Rereacts with Ta to form Re—Ta intermetallic compounds. Similar to Example2, these Re—Ta intermetallic compounds have a fine grained (<4 μm)randomly oriented surface structure, which block the originalpolycrystalline grain structure of the Ta substrate 112. Finally, the Taemissive layer 116 is added, as illustrated in FIG. 8B. The final Talayer serves as the emissive layer, because it has the lowest workfunction of the three. The grain structure of the Ta layer 116 is fineand randomized because of the underlying Re-Ta alloy 1162.

The material arrangements in Examples 1-3 have shown improved uniformemission characteristics in emission microscopes and SCALPEL™ machinescompared to the conventional Ta cathode.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A thermoionic cathode, comprising: a substrate;an emissive layer; and a buffer, located between said substrate and saidemissive layer, said buffer inhibiting interaction of said emissivelayer and said substrate by way of one of altering, and altering andblocking said substrate.
 2. The thermoionic cathode of claim 1, whereinsaid buffer alters said substrate by randomizing a crystallographicorientation of a grain structure at a surface of said substratecontacting said buffer.
 3. The thermoionic cathode of claim 2, whereinsaid buffer by altering miniaturizes grain sizes of grains at thesurface of said substrate contacting said buffer.
 4. The thermoioniccathode of claim 2, wherein said buffer alters the grain structure atthe surface of said substrate contacting said buffer by at least one ofdissolution, alloying, reaction, precipitation, and new phase formation.5. The thermoionic cathode of claim 4, wherein said buffer is a solidsolution buffer.
 6. The thermoionic cathode of claim 5, wherein saidbuffer includes at least two of the group consisting of Mo, W, Nb, V,Ir, Rb, and Ta.
 7. The thermoionic cathode of claim 6, wherein the solidsolution buffer includes molybdenum, tungsten and tantalum.
 8. Thethermoionic cathode of claim 7, wherein said substrate and said emissivelayer are made of tantalum.
 9. The thermoionic cathode of claim 1,wherein said cathode has a curved shape.
 10. The thermoionic cathode ofclaim 1, wherein said cathode is part of a projection electronlithography system.
 11. The thermoionic cathode of claim 10, wherein theprojection electron lithography system is a SCALPEL™ system.
 12. Thethermoionic cathode of claim 1, wherein said buffer is an alloyedbuffer.
 13. The thermoionic cathode of claim 12, wherein said buffer isan alloy comprising at least two elements with different crystallinestructure.
 14. The thermoionic cathode of claim 12, wherein said bufferis made of a grain growth inhibiting multiphase structure.
 15. Thethermoionic cathode of claim 1, wherein said buffer includes at leasttwo of Re, Ta, C, Hf, Te, Os, and Ru.
 16. The thermoionic cathode ofclaim 15, wherein the buffer includes rhenium and tantalum.
 17. Thethermoionic cathode of claim 16, wherein said substrate and saidemissive layer are made of tantalum.
 18. The thermoionic cathode ofclaim 1, wherein said buffer alters and blocks said substrate byreacting therewith to form a randomly oriented surface structure.